An illustrative configuration of a remote mirroring data facility is shown in FIG. 1. In the system shown in FIG. 1, a host CPU 1 is coupled to a source storage device 3 through a source storage controller 5. The storage device 3 may be a large disc drive providing main storage for the host CPU 1, or it may be a plurality of smaller disc drives arranged to perform the same function. An example of such a storage device is the SYMMETRIX line of disc arrays, available from EMC Corporation, Hopkinton, Mass.
The data stored in storage device 3 may be crucial to the operation of host CPU 1. Therefore, a contingency solution is desirable in the event that a problem is experienced with the storage device 3, to ensure that the stored data is not lost and to minimize the risk of CPU 1 being down due to problems with the storage device. Potential problems with the storage device 3 can include hardware and software errors that may make stored data unrecoverable, as well as catastrophic events, such as an earthquake or other natural disaster that could result in the destruction of the storage device.
One solution for protecting the data stored in storage device 3 is to mirror the data in another storage device. FIG. 1 illustrates such a system, wherein the source storage controller 5 is coupled to a target storage controller 7 and an associated target storage device 9. As data is written to the source storage device 3, it can also be written and mirrored to the target storage device 9. If the storage device 3 is destroyed or experiences an error that renders stored data unrecoverable, the data can be retrieved from the mirroring storage device 9. As shown in FIG. 1, the target storage controller 7 and target storage device 9 can also serve as the main storage system for another host CPU 11. Alternatively, the target storage system can be dedicated solely to mirroring the data stored in the source storage system, without being coupled to another CPU.
The level of protection provided by the system shown in FIG. 1 is enhanced when the mirroring target storage system (controller 7 and storage device 9) is disposed at a location remote from the source storage system (controller 5 and storage device 3), so that if a catastrophe occurs that destroys the source system, the likelihood of the mirroring storage system also being destroyed is greatly decreased. Thus, it is advantageous to implement a link 13 between the source and target storage controllers 5 and 7 that enables the target storage system to be disposed a relatively long distance away from the source storage system.
Communication between the host CPU 1 and the source storage controller 5 is typically performed using one of a number of standard protocols, including SCSI, BUS and TAG (B&T), or more typically ESCON. ESCON is a standard computer system interface and protocol developed by International Business Machines (IBM). ESCON defines the interface and protocol for communicating over a link 15 between the host CPU 1 and the source storage controller, and is described in detail, for example, in “ENTERPRISE SYSTEMS ARCHITECTURE/390—ESCON I/O INTERFACE”, 3rd Edition (1992) published by IBM. Because ESCON is used as the protocol to establish the interface between the host CPU 1 and the source storage controller 5, it has also conventionally been used to implement the link 13 between the source and target storage controllers 5 and 7 in a remote mirroring data facility such as the one shown in FIG. 1. Thus, link 13 has conventionally been implemented using a dedicated ESCON link that allows the target storage controller 7 to be disposed at a location somewhat remote from the source storage controller 5. However, these dedicated links have conventionally been limited to relatively short distances. For example, IBM states that its dedicated ESCON links are limited to approximately 60 km. Although this number may be conservative, it is believed that conventional dedicated ESCON links are limited to distances of approximately 80 km, even when appropriate repeaters are used along the line.
The use of a dedicated ESCON line to implement the link 13 between the source and target storage controllers 5 and 7 in a remote mirroring data facility is disadvantageous for two reasons. First, the above-described limit on the maximum supported distance may be insufficient for some applications. Second, it is often costly to implement these dedicated links because they are not part of a public communications network, and may not be capable of implementation over pre-existing communication lines.
To address the foregoing limitations on the use of a dedicated ESCON link to implement a remote mirroring data facility, the assignee of the present application has developed a technique for implementing a remote mirroring data facility using a link 13 implemented through pre-existing public communication channels, as disclosed in the following two U.S. patent applications, each of which is incorporated herein by reference: Ser. No. 08/601,733, entitled METHOD AND APPARATUS FOR INTERFACING TWO REMOTELY DISPOSED DEVICES COUPLED VIA A TRANSMISSION MEDIUM, filed Feb. 15, 1996; and Ser. No. 08/947,926, entitled METHOD AND APPARATUS FOR INTERFACING TWO REMOTELY DISPOSED DEVICES COUPLED VIA A TRANSMISSION MEDIUM, filed Aug. 25, 1997 (collectively assignee's co-pending applications).
The co-pending applications are directed to the implementation of the link 13 between the source and target storage controllers of a remote mirroring data facility via a data communication line operable in a public communications network. The specific embodiments discussed include data communication lines of the type leased by telephone service companies, such as data communication lines from the family of T-carriers available in North America (e.g., T1, T3 and T5) and the family of CEPT communication links available in Europe (e.g., CEPT-1 and CEPT-3). Leased lines such as a T-3 line are a service provided by telephone service companies in which a subscriber pays a flat fee to lease exclusive use of a data communication link between two locations. These leased lines employ existing telephone lines, satellite links, etc., to provide high speed data transmission within a public communications network between the two points requested by the subscriber. By implementing the link 13 in a remote mirroring data facility through a public communications network, the expense and time that would be incurred in implementing a conventional dedicated ESCON link between the source and target storage controllers 5 and 7 is avoided. Furthermore, the limitation on the distance over which dedicated ESCON links can extend is overcome, enabling the link 13 to be extended for significantly greater distances, thereby providing increased flexibility in implementing a user's desired configuration and providing enhanced protection in the event of a natural disaster or other catastrophe.
In the assignee's co-pending applications, a number of different protocols are specified for communicating between the source and target storage controllers 5 and 7 through the public communications network to achieve performance improvements as compared with the ESCON protocol. The use of a leased line and any one of these protocols achieves a high performance remote mirroring data facility. However, some user applications may not require the high performance achieved through the use of a leased line, and might desire a more inexpensive solution for implementing the link 13 between the source and target storage controllers in a remote mirroring data facility.
In view of the foregoing, it is an object of the present invention to provide an inexpensive link between two remotely disposed storage systems in a remote mirroring data facility.